The anode and cathode electrodes of a capacitor each possess inherent asymmetric electrically conducting properties and the two electrodes are series-opposed in the capacitor structure. Under the influence of a pulsating voltage, the electrodes charge and discharge alternately, that is, one of the electrodes charges as the other electrode discharges. As a consequence, the electrolyte between the electrodes is at a negative potential with respect to the charging electrode throughout each pulse. The relation governing the admittance of the capacitor [1/C (device) = 1/C (anode) + 1/C (cathode), where C is capacitance)] results in the electric charge transfer being limited by the smaller of the two capacitances, that is, either the capacitance of the anode electrode or the capacitance of the cathode electrode.
In polar electrolytic capacitors, particularly as to capacitor rating, the design thereof is preferably established by the design parameters of the anode electrode. Therefore, the capacitance of the cathode electrode should be orders of magnitude higher than the capacitance of the anode electrode so as to be compatible with the anode electrode design. If the capacitance of the cathode electrode is orders of magnitude higher than the capacitance of the anode electrode, the term 1/C (cathode) in the above relation becomes small relative to the capacitance of the anode electrode and the device capacitance is essentially equal to the anode electrode capacitance. Ideally, the operating characteristics of such a capacitor approach optimum stability as the capacitance of the cathode electrode approaches infinity.
One way to increase the capacitance of the cathode electrode of an electrolytic capacitor includes applying to the surface of that electrode a porous layer of finely divided, substantially inert conductive material such as carbon or a metal such as platinum, palladium or gold. The porous layer increases the surface area of cathode electrode and, hence, the capacitance of that electrode. However, it is presently thought that the charge-discharge current is conducted across the electrolyte which separates the cathode and anode electrodes by the agency of the dissociated ions of the inorganic electrolyte. For example: EQU H.sub.2 SO.sub.4 .fwdarw.2H.sup.+ + SO.sub.4.sup.-2 EQU hclO.sub.4 .fwdarw.H.sup.+ + ClO.sub.4.sup.- EQU hcl .fwdarw.H.sup.+ + Cl.sup.-
In operation of an electrical circuit, the anode electrode of an electrolytic capacitor is maintained at a positive DC bias voltage. This is necessitated by the aforementioned asymmetric character of the anode electrode. In the absence of a positive DC bias voltage negative ripple voltage pulses appear at the anode electrode which can cause current conduction through the relatively low forward resistance of this electrode. Thus, in practice, ripple voltages which are applied to a polar type electrolytic capacitor are superimposed on the positive DC bias voltage which the anode electrode sustains throughout electrical operation. This DC bias voltage, however, give rise to leakage current through defects in the anode dielectric layer and, generally speaking, the leakage current increases with increasing temperature and DC bias voltage. As leakage current is drawn through the series cathode electrode, certain electrochemical actions may occur at the cathode-electrolyte junction which can be detrimental to the overall device performance. For example, positive hydrogen ions may draw electrons from the cathode electrode to form neutral hydrogen gas bubbles some of which may cling to the cathode electrode to produce a layer of nonconducting gas around the cathode electrode. The base or basis metal surface of the cathode electrode may react with the negative ions of the inorganic electrolyte to form a neutral insoluble electrically insulating film such as silver sulfite (Ag.sub.2 SO.sub.3) or silver sulfide (Ag.sub.2 S) where the base metal surface of the cathode electrode is silver covered with a porous layer of platinum, or perhaps copper sulfide (CuS) where the base metal surface of the container is copper covered with a porous layer of platinum. The insoluble insulating film and the hydrogen gas interfer with the operation of the capacitor in several ways. First, the film and gas bubbles each act to reduce the effective area of the cathode electrode so that the capacitance of the cathode electrode is undesirably reduced. Second, the insulating film on the cathode electrode harmfully increases the internal series resistance of the capacitor.
To help prevent the formation of an insoluble electrically insulating film and/or hydrogen gas at the cathode electrode, a depolarizer is added to the electrolyte. The aforementioned current flow at the cathode-electrolyte interface causes the depolarizer to be reduced prior to reduction of the inorganic acid of the electrolyte. The reduction of the depolarizer occurs prior to the reduction of the inorganic acid of the electrolyte because the decomposition potential of the metal ion of the depolarizer is lower than that of the hydrogen ion of the inorganic acid. For example, a silver sulfate depolarizer (Ag.sub.2 SO.sub.4) in a sulfuric acid electrolyte has its molecule broken or reduced into positive silver ions (2Ag.sup.+) and negative sulfate ions (SO.sub.4.sup.-2) by the flow of current across the electrolyte-cathode interface prior to current flow breaking or reducing the sulfuric acid molecule of the electrolyte into positive hydrogen ions (2H.sup.+) and negative sulfate ions (SO.sub.4.sup.-2). Positive silver ions are deposited on the cathode electrode as silver metal or dissolved therefrom as silver ions depending on the direction of current flow at the electrolyte-cathode interface. In any regard, as long as silver sulfide depolarizer is present at the electrolyte-cathode interface, no electrolyte will be reduced to form harmful gaseous or film products on the cathode electrode surface because reduction of the depolarizer takes place prior to reduction of the electrolyte.